How does a developing tissue know how much to grow and when to stop? On p. , Marcos Gonzalez-Gaitan and colleagues address this question using the Drosophila eye as a model. This study follows their earlier work proposing a temporal model for growth control in the wing, whereby cells divide when the levels of Decapentaplegic (Dpp) signalling increase by a defined percentage. In the eye, spatial growth patterns are very different from those in the wing, and growth is partially dependent on a Dpp gradient, the source of which – the morphogenetic furrow – moves as development progresses. The authors find that, as in the wing, the signal gradient scales with tissue size – which grows and then shrinks with the progression of the furrow. They then show that their temporal model is quantitatively consistent with observed patterns of proliferation in wild-type and in various mutant conditions. Intriguingly, they also show that the Dpp-independent component of growth control can be explained by a temporal model – implying a similar cellular response to a different signalling gradient. Thus, a model of tissue growth that involves cells dividing in response to defined increases in signalling levels may be applicable across multiple tissues and multiple signalling inputs.
